Getting wordpress error: Cannot modify header information

I'm getting this error (I activated wp-debug) when I try to activate a theme or when I try to save a post:

Warning: Cannot modify header information - headers already sent by (output started at /mnt/webv/b3/13/56920413/htdocs/WordPress_02/wp-content/themes/BackpackFamily/functions.php:58) in /mnt/webv/b3/13/56920413/htdocs/WordPress_02/wp-admin/post.php on line 197

Warning: Cannot modify header information - headers already sent by (output started at /mnt/webv/b3/13/56920413/htdocs/WordPress_02/wp-content/themes/BackpackFamily/functions.php:58) in /mnt/webv/b3/13/56920413/htdocs/WordPress_02/wp-includes/pluggable.php on line 1171

Also, be sure there's no stray line feeds before the opening <?php tag.

Side-comment:
This is a NOTICE, not really an error. If you had debug turned off, it would be a non-issue because notices would be silenced.
BUT...Kudos to you for testing your code with WP_DEBUG set to true. This is the only way to be sure your code runs without notices!